Coated columbium



United States Patent 3,081,530 CUATED CDLUMBEUM Stanley T. Wlodelr,Niagara Falls, N.Y., assignor to Union Iarbide Corporation, acorporation of New York No Drawing. Filed Aug. 3, 1960, Ser. No. 47,1367 Claims. (1. 29-194) This application is a continu-ationin-part ofapplication Ser. No. 781,839 filed December 22, 1958.

The present invention relates to coated colu-mbium and to a method forcoating columbiurn to obtain columbium articles characterized byhigh-temperature oxidation resistance and resistance to thermal andmechanical shock.

Jet aircraft, rockets, missiles and the like require hightemperaturematerials of construction which have good oxidation resistance and goodthermal and shock resistance at elevated temperatures. The number ofmaterials which are characterized by these properties and which aresuitable for the rigorous treatment to which these materials will besubjected are very few. Among the most promising are the refractorymetals and alloys thereof such as tungsten, molybdenum, tantalum andcolumbium. Without exception, however, these materials are rapidlyoxidized far below the elevated temperatures required in service, i.e.,about 1700" F. to 2000 F. and above.

These refractory materials and alloys to be useable must also beworkable. However, it is a well established fact that workability andoxidation resistance do not generally occur simultaneously, i.e., thehigher the oxidation resistance the lower is the workability of thematerial in most cases.

One method which has been employed to overcome these problems is toapply a protective coating to these materials. The requirements forturbine buckets to operate around 2000 F. illustrate the nature of theproblems encountered. The coating must withstand oxidation in combustiongases. It must be free of defects and/or be self-healing beforedestructive oxidation occurs, i.e., it must be able to provide its ownprotective coating in the oxidizing atmosphere in the event any defectsdo occur in the coating. The coating must Withstand the stress inducedwih thermal gradients. It must have high resistance to thermal shock astemperatures may vary as much as 1000 F. in a matter of seconds. Thecoating must resist severe fatigue stresses and possess sufficientductility to elongate without failure. It must resist mechanical shockand especially foreign particles entrained in a gas stream. It mustwithstand the corrosive and erosive action of the gas stream itself.While these prob lems were stated to exist with coated turbine bucketsequivalent or similar problems and exacting requirements are present inother components of jet engines, missiles, rockets, etc.

Many prior compositions have been tried as coating materials forrefractory metals, for example, molybdenum disilicide, ceramic-typecoatings, enamels, chrome coatings, nickel-chrome alloys, alloys ofaluminum, nickel and silicon and nickel-boron. All are known to failwhen subjected to severe tests requiring the performances describedabove. In general, the more ductile tough coatings are not sufiicientlyoxidation resistant or have too low melting points to be useful whereasthose possessing good oxidation resistance do not possess self-healingproperties and/or are too brittle and sensitive to impact.

One of the most common causes of failure occurs when the coating obtainspin-point defects and cracks as a result of thermal stresses. If thecoatings exhibited selfhealing characteristics prior to the destructiveoxidation of the base metal such failures could be prevented. Many ofthe known coatings described above exhibit self-healing but likemolybdenum disilicide will not self-heal at low temperatures. Molybdenumdisilicide, MoSi does not effectively heal below 2200 F. to 2500 F. Itis, therefore, obvious that such a coating would not be suitable belowsuch a temperature as small defects develop. A coating that self-healsat relatively low temperatures, 1200 F. to 1700 F., has many advantagesand is, therefore, desirable.

Accordingly, it is an object of the present invention to provide acoated refractory metal, which coated material is able to withstandsevere thermal and mechanical stresses under oxidizing conditions.

Another object of the invention is to provide a coated refractory metalhaving thermal shock resistance, mechanical shock resistance andexhibiting self-healing characteristics.

Still another object of the present invention is to provide a processfor improving the properties of columbium and columbium alloys.

The objects are achieved by coating columbium metal and columbium-basealloys with a plurality of coatings at least one of which ischaracterized by high-resistance to thermal and mechanical shock and thesecond being characterized by oxidation resistance.

The internal layer which is contiguous to the metal being coated must bethe coating exhibiting thermal and mechanical shock resistance and mustbe miscible with the metal being protected. This coating should beplastic, and should exhibit a coeflicient of expansion and a modulus ofelasticity substantially the same as that of the base metal beingprotected. The exterior coating must be oxidation resistant and exhibitself-healing characteristics.

It is possible to employ more than a single coating of each of theoxidation resistant and shock resistant materials but in any case theshock resistant layer should be contiguous to the material being coatedand the exterior coating must be an oxidation resistant coating. Thephysical properties of the exterior coating and the interior coatingshould correspond so that residual strains that would normally exist atthe interface between a coating and the metal being coated aresubstantially eliminated or at least greatly alleviated by the presenceof an intermediate or inner layer.

An outer and oxidation resistant coating which is suitable for use inthe present invention comprises an alloy of molybdenum, silicon,chromium, boron and aluminum. This alloy more specifically comprisesbetween 10 and 40 atom percent of at least one metal selected from thegroup consisting of molybdenum, tungsten, tantalum, columbium, andvanadium, between 20 and 65 atom percent silicon, between 2 and 16 atompercent of at least one metal selected from the group consisting ofchromium, titanium and zirconium, between 2 and 25 atom percent boronand between 3 and 30 atom percent aluminum. The remainder of thecomposition is oxygen and other impurities such as carbon.

It has been found that a boron constituent and the constituent of thesecond metal grouping above (chromium, titanium or zirconium) can existin the composition of the invention in the form of a metal boride, as amixture of metal borides or as a mixture of elemental metals and boron.

A very suitable composition has been prepared from 40 percent by weightof molybdenum, 40 percent by weight silicon, 10 percent by weightchromium boride, which may be represented by Cr B and 10 percent byweight of aluminum. In atomic percentages this is 18.3 percent Mo, 62.3percent Si, 3.2 percent Cr B and 16.3 percent Al. This composition maybe made as a blend of powders, or preferably as a prealloyed powder. Thealloy offers greater uniformity when it is used for coating purposes.The optimum raw material composition appears to be noted above, namely18.3 percent Mo62.3 percent Si3.2 percent Cr B 16.3 percent Al, inatomic percentages, although protective coatings have been made fromcompositions in the range of 30-65 percent Si, 10-35 percent Mo, 216percent Cr, 225 percent B and 5-30 percent Al.

This composition of matter may be applied by the detonation coatingmethod described in US. Patent 2,714,563 issued to R. M. Poorman et al.on August 2, 1955. In that process, a powdered composition to be coatedis suspended in a body of detonatable gas in an elongated barrel capableof sustaining a detonation, and, upon ignition of the body ofdetonatable gas, the suspended powder is ejected from the barrel underthe impetus of the detonation and directed against the surface of thebody to be coated.

Coatings may be provided utilizing the composition of matter inconjunction with other known flame spraying processes such as theWall-Colmonoy process.

A spray gun employing an oxy-acetylene flame as the heat source may beused in this coating process. The fuel-gas mixture is adjusted toproduce essentially a chemically neutral flame. Powder consisting incomposition of 40 weight percent Mo, 40 weight percent Si, weightpercent Cr B and 10 weight percent Al is aspirated into the flame zoneof the spray gun by means of an argon stream passing through a powderdispenser. The outlet of the spray gun is held about 6 inches from theworkpiece. The workpiece is rotated and the spray gun is traversed alongthe axis of the workpiece so as to apply a coating 0.008-inch thick. Thecoating produced by this method is characteristically porous and furtherheat treatment is necessary in order to obtain a satisfactory coating.The coated workpiece may be placed in a furnace and heated to 1lOO C.for 3 hours in a hydrogen atmosphere.

In addition, dipping, painting, or spraying the refractory body with aslurry of the suspended alloy or blended powder followed by heattreatment in an inert or reducing atmosphere may be employed to providecoatings from the novel composition of the invention.

It has been found that the detonation process of applying the novelcomposition of matter as a coating for surfacing bodies offers manyadvantages.

Suitable inner coatings are composed of columbiumbase alloys with atleast one of the metals of the group consisting of titanium, chromium,vanadium, aluminum, nickel and iron. Fair examples of suitable innercoating alloys can be specified.

The first of the aforementioned suitable inner alloy coating is acomposition consisting essentially of from -1 to 40 weight percent oftitanium, 8 to 30 weight percent iron, 3 to 35 Weight percent chromium,up to 10 weight percent vanadium, up to 30 weight percent tungsten, upto 30 weight percent tantalum, the aggregate of said vanadium, tungstenand tantalum not exceeding 50 weight percent, up to weight percent inthe aggregate of at least one metal selected from the group consistingof nickel and cobalt, up to 5 weight percent in the aggregate of atleast one alloying element selected from the group consisting of barium,silicon, beryllium, yttrium, boron and the rare earth metals, theremainder being columbium in a minimum amount of at least 30 weightpercent.

The second of the aforementioned suitable inner alloy coating is acomposition consisting essentially of from 1 to 40 weight percenttitanium, 1 to 30 weight percent chromium, 1 to 40 weight percentaluminum, 0.5 to 10 weight percent vanadium, up to 30 weight percent inthe aggregate of at least one metal selected from the group consistingof tungsten and tantalum, up to 10 weight percent in the aggregate of atleast one metal selected from the group consisting of manganese, nickel,iron, cobalt, zirconium and hafnium, up to 5 weight percent in theaggregate of at least one alloying element selected from the groupconsisting of barium, silicon, beryllium, yttrium, boron, and the rareearth metals,

and the remainder being columbium in an amount of at least weightpercent.

The third of the aforementioned suitable inner alloy coating acomposition consisting essentially of a minimum of 27 weight percentcolumbium, 10 to 50 weight percent tungsten, 5 to 40 weight percenttantalum, about 0 to 20 weight percent titanium, about 0 to 20 weightpercent chromium, about 0 to 7 weight percent vanadium, about 0 to 5Weight percent iron, about 0 to 5 weight percent nickel, about 0 to 7weight percent aluminum, about 0 to 5 weight percent cobalt, about 0 to2 weight percent beryllium, about 0 to 5 weight percent zirconium, about0 to 5 weight percent hafnium, about 0 to 2 weight percent barium, about0 to 2 weight percent thorium, about 0 to 2 weight percent yttrium,about 0 to 2 weight percent of at least one rare earth metal, the sumtotal of titanium, chromium, aluminum, iron, nickel cobalt and vanadiumnot exceeding 50 weight percent and the sum total of zirconium, hafnium,barium, beryllium, yttrium, thorium and the rare earth metals notexceeding 6 Weight percent; and incidental impurities.

The fourth of the aforementioned suitable inner alloy coating acomposition consisting essentially of a minimum of weight percentcolumbium, about 0 to 60 weight percent zirconium, about 0 to 10 weightpercent of aluminum, about 0 to 10 weight percent vanadium, about 0 to20 weight percent titanium, about 0 to 5 weight percent beryllium, about0 to 5 weight percent magnesium, about 0 to 5 weight percent silicon,about 0 to 5 weight percent barium, about 0 to 5 weight percent tin,about 0 to 5 weight percent of at least one rare earth metal andincidental impurities.

Other suitable coatings are described in US. Patent No. 2,822,268 toHix, US. Patent No. 2,838,395 to Rhodiu and US. Patent No. 2,838,396 toRhodin. An inner coating which has been found to be quite suitablecomprises from 30 percent to 60 percent columbium, 10 percent to 40percent titanium, 5 percent to 15 percent chromium, 2 percent to 10percent aluminum, up to 10 percent vanadium, up to 20 percent nickel andup to 20 percent iron and preferably from 40 to percent columbium, 20percent to 40 percent titanium, 8 percent to 15 percent chromium, 3percent to 7 percent aluminum, up to 7 percent vanadium, up to 15percent nickel and up to 15 percent iron.

The various coatings may be applied to the columbium metal being coatedby a method such as that described in U.S. Patent No. 2,714,563 toPoorman et al. However, other methods, e.g., spraying, flame spraying,flame plating, pressure bonding, etc., may be utilized to obtain thesecoatings.

The inner alloy may be applied in substantially the same manner as thatdescribed above for the preparation of the outer coating. To obtain thepowdered material the alloy composition for the inner coating may beprepared by non-consumable arc-melting procedures after which the ingotsso obtained may be embrittled by treatment in pure hydrogen at 600 F.The embrittled ingots may be crushed to through 325 mesh. The powder maybe de'hydrogenated by vacuum annealing.

To illustrate the advantages obtained by employing the multiple coatingof the present invention rods of pure columbium metal 2 inches long andA inch in diameter having hemispherical ends were coated either with theoxidation resistant coating alone, or the oxidation resistant coatingwith the inner shock resistant coating, both being coated by the methodsdescribed previously. The specimens were employed in tests to determinetheir selfhealing and shock resistant properties. The properties of thesingle coated materials were compared to those of the dual coatedmaterials. The results are described in the following examples.

Example I A pure columbium metal rod, prepared in the manner describedpreviously, was coated by flame-plating tech? niques with a 0.008-inchlayer of an alloy consisting of 40 percent molybdenum by weight, 40percent silicon, 7 .6 percent chromium, 2.4 percent boron, and percentaluminum. An identical rod was coated in the same manner first with an0.005-inch layer of an alloy consisting of 46 percent by weightcolumbium, 30* percent titanium, percent chromium, 5 percent vanadium, 4percent aluminum, and, subsequently, with an 0.0035- inch layer of analloy consisting of 40 percent by weight molybdenum, 40 percent silicon,7.6 percent chromium, 2.4 percent boron, and 10 percent aluminum. 030thspecimens withstood 1000 hours exposure in air at 1150 C. withoutfailure. After this preliminary test, four holes, 0.040, 0.031, 0.02 and0.0135 inch in diameter and of an inch deep were drilled in eachspecimen. The two specimens were then exposed to the atmosphere at 1050C. with the result that the specimen which had been protected by thesingly molybdenum-silicon-ohromiumboron-aluminum coating sutferedprogressive oxidation and failure at the points where the coating hadbeen damaged by drilling, While the specimen treated with the duplexcoating showed remarkable selfi-healing tendency to the point thatlittle visible attack occurred after 120 hours of exposure at 1050 C.

Exam'ple 11 Four pure columbium metal rods, prepared in the mannerdescribed previously, were used in this experiment. Two of the rods werecoated by flame-plating techniques with an 0.008-inch layer of an alloyconsisting of 40 percent by weight molybdenum, 40 percent silicon, 7.6percent chromium, 2.4 percent boron, and 10 percent aluminum, and theother two rods were coated by the same method with a first layer, 0.004inch thick, of an alloy consisting of 47 percent by weight columbium, 30percent titanium, 10 percent chromium, 10 percent nickel, 3 percentaluminum, and, subsequently, with a second layer, 0.003 inch thick, ofan alloy consisting of 40 percent by weight molybdenum, 40 percentsilicon, 7.6 percent chromium, 2.4 percent boron, and 10 percentaluminum. The four specimens were exposed to an oxidizing atmosphere(air) at 1150 C. and periodically removed from said hot environment andimmediately quenched in water. The two specimens coated with a singlelayer were able to withstand only 3 and 5 water quenches, respectively,during a total exposure of 140 and 150 hours, respectively, while theremaining specimens protected by the duplex coating withstood 30 and 24water quenches, respectively, during a total exposure of 267 and 188hours, respectively.

Example 111 A columbium rod was flame coated with a first coatingcomprising 32.3 weight percent titanium, 14.9 weight percent chromium,4.84 weight percent vanadium, 4.24 weight percent aluminum, balancecolumbium and remainder incidental impurities and a second exteriorcoating comprising 40 atomic percent molybdenum, 40 atomic percentsilicon, 8 atomic percent chromium, 2 atomic percent boron and 10 atomicpercent aluminum. The resultant coated columbium exhibited an oxidationresistance of 1050 hours at a temperature of 1150 C. and withstood 7water quenches from a temperature of 1150 C. before failure.

Example IV A columbium rod was flame coated with a first coatingcomprising 31.1 weight percent titanium, 10.2 weight percent chromium,9.9 weight percent nickel, 3.2 weight percent aluminum, balancecolumbium and remainder incidental impurities including iron and asecond exterior coating comprising 40 atomic percent molybdenum, 40atomic percent silicon, 8 atomic percent chromium, 2 atomic percentboron and 10 atomic percent aluminum. The resultant coated columbiumwithstood 30 water quenches from a temperature of 115 0 C. beforefailure.

Example V A columbium rod was flame coated with a first coatingcomprising 30.4 weight percent titanium, 9.8 weight percent chromium,11.5 weight percent iron, 3.1 weight percent aluminum, balance columbiumand remainder incidental impurities and a second exterior coatingcomprising 40 atomic percent molybdenum, 40 atomic percent silicon, 8atomic percent chromium, 2 atomic percent boron and 10 atomic percentaluminum. The resultant coated columbium withstood 21 Water quenchesfrom a temperature of 1150 C. before failure.

Example VI A columbium rod was flame coated with a first coatingcomprising 28.7 weight percent titanium, 11.1 weight percent chromium,9.3 weight percent nickel, 3.4 weight percent aluminum, balancecolumbium remainder incidental impurities and a second exterior coatingcomprising 40 atomic percent molybdenum, 40 atomic percent silicon, 8atomic percent chromium, 2 atomic percent boron and 10 atomic percentaluminum. The resultant coated columbium exhibited an oxidationresistance of 672 hours at a temperature of 0 C.

As may be seen from the foregoing examples, the dual coated materialsexhibit far greater self-healing characteristics and shock resistancethan the single coated materials.

What is claimed is:

1. A process for the production of duplex-coated refractory metalscomprising selecting at least one refractorymetal base from the groupconsisting of columbium and columbium alloys; providing and applying tosaid refractory-metal base a 'first coating contiguous to and misciblein said base, said first coating and said base exhibiting substantiallysimilar coefficients of expansion and moduli of elasticity, said firstcoating comprising from 30 to 60 weight percent columbium, 10 to 40weight percent titanium, 5 to 15 weight percent chromium, 2 to 10 weightpercent aluminum, up to 10 weight percent vanadium, up to 20 Weightpercent nickel, up to 20 weight percent iron, and incidental impurities;and providing and applying a second coating, said second coatingcomprising between 10 and 40 atom percent of at least one metal selectedfrom the group consisting of molybdenum, tungsten, tantalum, columbiumand vanadium, between 20 and 65 atom percent silicon, between 2 and 16atom percent of at least one metal selected from the group consisting ofchromium, titanium and zirconium, between 2 and 25 atom percent boron,between 3 and 30 atom percent aluminum, the re mainder being incidentalimpurities; said second coating being characterized by self healingproperties and oxidation resistance.

2. A duplex coated columbium-base metal product characterized byresistance to severe thermal and mechanical stresses,oxidation-resistance, and self-healing, which comprises; arefractory-metal base selected from the group consisting of columbiumand columbium-base alloys; a first coating contiguous to said basecomprising 30 to 60 Weight percent columbium, 10 to 40 weight percenttitanium, 5 to 15 weight percent chromium, 2 to 10 weight percentaluminum, up to 10 weight percent vanadium, up to 20 weight percentnickel, up to 20 weight percent iron, and the remainder incidentalimpurities, said first coating being miscible in said refractory-metalbase and having substantially similar coeificients of expansion andmoduli of elasticity as said refractory-metal base; and a second coatingcomprising between 10 and 40 atomic percent of at least one metalselected from the group consisting of molybdenum, tungsten, tantalum,columbium and vanadium, between 20 and 65 atomic percent silicon,between 2 and 16 atomic percent of at least one metal selected from thegroup consisting of chromium, titanium and zirconium, between 2 and 25atomic percent boron, between 3 and 30 atomic percent aluminum and theremainder incidental impurities.

3. A duplex coated columbium-base metal product in accordance with claim2 wherein said refractory-metal base is essentially columbium.

4. A duplex coated columbium-base metal product in accordance with claim2 wherein the first coting contiguous to said base is composed of 40 to50 weight percent columbium, 20 to 40* Weight percent titanium, 8 to 15weight percent chromium, 3 to 7 weight percent aluminum, up to 7 Weightpercent vanadium, up to 15 weight percent nickel, up to 15 Weightpercent iron and the re mainder incidental impurities,

A duplex coated columbium-base metal product characterized by resistanceto severe thermal and mechanical stresses, oxidation-resistant andself-healing, which comprises; an essentially columbium refractory-metalbase; a first coating contiguous to said base consisting essentially of46 weight percent columbium, 30 weight percent titanium, weight percentchromium, 4 weight percent aluminum and 5 weight percent vanadium,wherein said first coating is miscible in said refractory metal base;and a second exterior coating consisting essentially of 40 atomicpercent molybdenum, 40' atomic percent silicon, 7 .6 atomic percentchromium, 2.4 atomic percent boron and 1() atomic percent aluminum, saidfirst coating and said columbium refractory-metal base exhibitsubstantially similar coefficients of expansion and moduli ofelasticity.

6. A duplex coated columbium-base metal product characterized byresistance to severe thermal and mechanical stresses, oxidationresistant and self-healing, which comprises; an essentially columbiumrefractory-metal base; a first coating contiguous to said baseconsisting essentially of 47 Weight percent columbium, 30 weight percenttitanium, 10 weight percent chromium, 3 weight percent aluminum nad 10weight percent nickel; and a second exterior coating consistingessentially of atomic percent molybdenum, 40 atomic percent silicon, 7.6atomic percent chromium, 2.4 atomic percent boron and 10 atomic percentaluminum, wherein said first coating and said columbium refractory-metalbase exhibit substantially similar coefficients of expansion and moduliof elasticity.

7. A duplex coated columbium-base metal product characterized byresistance to severe thermal and mechanical stresses, oxidationresistant and self-healing, which comprises; an essentially columbiumrefractory-metal base; a first coating contiguous to saidbase consistingessentially of 44 weight percent columbium, 36 weight percent titanium,10 weight percent chromium, 4 weight percent aluminum and 12 weightpercent iron; and a second exterior coating consisting essentially of 40atomic percent molybdenum, 40 atomic percent silicon, 8 atomic percentchromium, 2.0 atomic percent boron and 10 atomic percent aluminum,wherein said first coating and said columbium refiractory-metal baseexhibit substantially similar coefiicients of expansion and moduli ofelasticity.

References Cited in the file of this patent UNITED STATES PATENTS2,472,930 Wilkes June 14, 1949 2,690,409 Wainer Sept. 28, 1954 2,763,919Kempe Sept. 25, 1956 2,871,150 Fraser Ian. 27, 1959

2. A DUPLEX COATED COLUMBIUM-BASE METAL PRODUCTS CHARACTERIZED BYRESISTANCE TO SEVERE THERMAL AND MECHANICAL STRESSES,OXIDATION-RESISTENCE, AND SELF-HEALING, WHICH COMPRISES; AREFACTORY-METAL BASE SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM ANDCOLUMBIUM-BASE ALLOYS; A FIRST COATING CONTIGUOUS TO SAID BASECOMPRISING 30 TO 60 WEIGHT PERCENT COLUMBIUM, 10 TO 40 WEIGHT PERCENTTITANIUM, 5 TO 15 WEIGHT PERCENT CHORMIUM, 2 TO 10 WEIGHT PERCENTALUMINUM, UP TO 10 WEIGHT PERCENT VANADIUM, UP TO 20 WEIGHT PERCENTNICKEL, UP TO 20 WEIGHT PERCENT IRON, AND THE REMAINDER INCIDENTALIMPURTIES, SAID FIRST COATING AND THE REMAINDER INCIDENTAL IMPURITIES,SAID FIRST COATING BEING MISCIBLE IN SAID REFRACTORY-METAL BASE ANDHAVING SUBSTANTIALLY SIMILAR COEFFICIENTS OF EXPANSION AND MODULI OFE;ASTICITY AS SAID REFRACTORY-METAL BASE; AND A SECOND COATINGCOMPRISING BETWEEN 10 AND 40 ATOMIC PERCENT OF AT LEAST ONE MATALSELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN, TATALUM,COLUMBIUM AND VANADIUM, BETWEEN 20 AND 65 ATOMIC PERCENT SILICON,BETWEEN 2 AND 26 ATOMIC PERCENT OF AT LEAST ONE METAL SELECTED FROM THEGROUP CONSISTING OF CHROMIUM, TITANIUM AND ZIRCONIUM, BETWEEN 2 AND 25ATOMIC PERCENT BORON, BETWEEN 3 AND 30 ATOMIC PERCENT ALUMINUM AND THEREMAINDER INCIDENTAL IMPURITIES.